1. Field of the Invention
The present invention relates generally to high-frequency power transistors, and specifically to monolithic microwave integrated circuit (MMIC) amplifiers.
2. Description of Related Art
High-frequency power transistors have traditionally occupied a large amount of the area available on a monolithic integrated circuit. The higher the output power requirements, the larger the gate width of the power transistor. For example, the gates of metal semiconductor field effect transistors (MESFETs) often exceed one millimeter in total periphery, and can extend across the entire width of the integrated circuit.
Thus, high power monolithic microwave integrated circuit (MMIC) amplifiers typically become very wide and take on a rectangular aspect ratio, which may reach width-to-length ratios of 4:1 or higher. These high aspect ratio MMICs are subject to higher stress levels during wafer processing, wafer handling, die separation and MMIC assembly. In addition, semiconductor materials, such as gallium arsenide (GaAs), are typically brittle, which results in lower overall MMIC yields due to die cracking.
Wide MMIC amplifiers also create significant packaging and housing problems at mm-wave frequencies. The wide MMIC amplifiers must be mounted in wide waveguide cavities, which may allow higher order modes of electromagnetic wave propagation, leading to additional radiation losses, coupling problems from one circuit to another, resonances and isolation problems that can create amplifier instabilities (i.e., spurious responses or oscillations).
Traditionally, placement of large gate width cells on both the horizontal and vertical (x, y) edices of GaAs MMICs has been restricted because GaAs requires that all FET gates (e.g., FET channels and gate fingers) on a MMIC amplifier must run in the same direction. Since GaAs etches in an anisotropic way, all gates on a GaAs MMIC must be placed in one direction. Therefore, MMICs with the same physical area, but with smaller aspect ratios, have not been able to be designed.
Small aspect ratio MMICs would be advantageous since fewer high aspect ratio MMICs can be placed on a fixed diameter wafer than MMICs with the same area, but with smaller aspect ratios. Therefore, the yield loss on a wafer implementing high aspect ratio MMICs may range from less than one percent to several percent, depending upon die area and differences in aspect ratios. As a result, small aspect ratio MMICs are less expensive to produce than MMICs with the same area, but with larger aspect ratios.
A small aspect ratio, high power MMIC amplifier is disclosed. The small aspect ratio MMIC amplifier is capable of achieving the same power levels as conventional power amplifier designs, but with an aspect ratio of near 1:1, versus 4:1 of conventional power amplifiers. The small aspect ratio MMIC is narrower than conventional high power amplifiers, simplifying handling, assembly, packaging and housing issues, and greatly reducing the chances of unwanted resonances or instabilities for the assembled circuit in complex multi-chip modules.
The small aspect ratio MMIC power amplifier layout uses two different types of FETs, with all gate fingers of both types of FETs running in the same direction. One type of FET is a conventional FET, in which the gate fingers run parallel to the direction of the output. In the conventional FET, the gate manifold and the drain manifold both generally extend in the x-direction (parallel to each other). The other type of FET has gate fingers that run perpendicular to the direction of the output. In this other type of FET, the gate manifold generally extends in the x-direction, while the drain manifold generally extends in the y-direction (perpendicular to each other). By using two different types of FETs, large gate width power FETs can be placed on two, three or four sides of the MMIC, versus the conventional power amplifier layout where large gate width FETs are placed in parallel and run along or near one edge (side) of the MMIC amplifier.
The small aspect ratio MMIC power amplifier layout also reduces the width of external divider and combiner circuitry, which is often required to achieve very high power levels from single or multiple MMIC amplifiers. Furthermore, the small aspect ratio MMIC amplifier layout allows more MMICs to be placed in fixed width transmitter modules, enabling higher transmitter power levels to be achieved.